The invention relates to a surgical instrument for the mechanical removal of bone cement according to the preamble of claim 1, and a process for the production of shock waves adapted for the removal of bone cement according to the preamble of claim 16.
Surgical instruments of the above type are required in orthopedic operations, for instance when after the removal of a hip joint prosthesis the remaining bone cement, e.g. made from PMMA, has to be removed, and for providing an accurate lodgment of the newly inserted hip joint prosthesis. For removing the cement, use is made e.g. of chisel tools which are driven into the cement with the aid of a hammer to thus split off the cement from the bone in the bone-marrow cavity.
The removal of bone cement is largely required in orthopedic surgery in order to exchange or consolidate date endoprostheses which have become loose or infected. Also other applications are possible.
Particularly in hip endoprosthetics, prostheses are exchanged with increasing frequency. In prostheses fixed by cement, the removal of the bone cement is bothersome and time-consuming. As of yet, the removal of the cement is performed by chisels of various shapes; however, such chisels pose problems as to their safe use, e.g. in the depth of the marrow cavity where visual inspection is difficult. Besides the large time requirements, damage may be caused to the bones, rendering the new implantation of a prosthesis impossible or resulting in an excessive loss of bone substance.
Further, pneumatic hammers are known (EP 0 144 005, W095/22934) wherein a piston element is pneumatically reciprocated in a cylinder, wherein, at the distal end of the cylinder, the piston element exerts an impact onto a chisel tool axially supported in the housing. In such a system, the chisel tool is accelerated to speeds slightly above 3 m/s, and the stroke of the chisel tool is about 8 mm. The large stroke entails the danger that the chisel tool accidentally penetrates into the bone substance.
Thus, the above surgical instrument is merely suited to imitate the heretofore manually performed blow of a hammer. However, it has been found that the efficiency of such a pneumatically operated chisel does not meet the requirements in the removal of bone cement and involves the risk of damage to the bone.
Further known surgical instruments are operable to melt and remove thermoplastic bone cement by means of ultrasonic vibrations (U.S. Pat. No. 5,221,282). Using such an instrument, the bone cement will become ductile from about 100.degree. C. and then can be removed. In the process, temperatures up to 200.degree. C. are generated at the tip of the tool and may cause damage to the bone. The smoke generated during the removal of bone cement produces released monomers having a toxic effect.